Top UN officials condemn unrelenting bombardment of Syrian enclave as air attacks continue and death toll nears 300.

The US ambassador to the UN has joined the chorus of voices calling for an end to human suffering in the besieged Syrian rebel enclave of Eastern Ghouta.

In a statement on Wednesday, Nikki Haley urged the Security Council to pass a draft resolution calling for a 30-day ceasefire in Eastern Ghouta.

"It’s time to take immediate action in the hopes of saving the lives of the men, women, and children who are under attack by the barbaric Assad regime," she said.

She also called for the ceasefire to allow aid to be delivered to Eastern Ghouta.

The remarks came during continued shelling of the Damascus suburb. The Syrian Observatory for Human Rights said warplanes and helicopters were grounded on Thursday morning due to bad weather, but shelling from "regime forces" killed at least 13, including three women and three children.

At least 335 people have been killed since February 18, according to SOHR.

Separately, top UN officials demanded an immediate cessation of aerial attacks on Eastern Ghouta, with Antonio Guterres, the secretary-general, calling the place "hell on Earth".

"My appeal to all those involved is for an immediate suspension of all war activities in Eastern Ghouta," Guterres told a UN Security Council meeting.

"This is a human tragedy that is unfolding in front of our eyes, and I don't think we can let things go on happening in this horrendous way."

The Security Council is expected to vote on the draft resolution as soon as Thursday.

About 400,000 people live in Eastern Ghouta, which has been besieged for years by government forces with few supplies reaching the desperate population.

UN human rights chief Zeid Ra'ad al-Hussein called the air attacks and artillery fire a "monstrous campaign of annihilation.

"International humanitarian law was developed precisely to stop this type of situation, where civilians are slaughtered in droves in order to fulfill political or military objectives," he said in a statement.

The UN has documented 346 civilian deaths with 878 wounded in Eastern Ghouta since February 4, mostly from air attacks hitting residential areas, Zeid said.

Russia denied involvement in air attacks on Wednesday, calling an accusation by a US official that Moscow was taking part in the raids "groundless".

"It is not clear what they are based on ... We do not agree," Russian presidential spokesman Dmitry Peskov said.

Russian jets

Russian jets have carried out air attacks in Syria in support of President Bashar al-Assad since 2015 - greatly changing the course of the seven-year war in the government's favour.

Negotiations to try to peacefully resolve the dire situation in Eastern Ghouta broke down after rebels there ignored calls to cease their resistance and lay down their arms, the Russian military said late on Wednesday.

Rebels were preventing civilians from leaving the conflict zone, Russia's ceasefire monitoring centre in Syria, which is run by the Russian military, also said in a statement.